Variable coupler fiberoptic sensor and sensing apparatus using the sensor

ABSTRACT

A variable coupler fiberoptic sensor utilizes a coupler having a fused coupling region that can be deflected to change the light distribution to a plurality of output fibers without putting the coupling region under tension. A compact, rugged, and highly sensitive sensor design is achieved by use of a coupler having a fused coupling region arranged substantially in a U-shape to allow the input and output fiberoptic leads to extend from the same side of the sensor structure.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] The U.S. Government has a paid-up license in this invention andthe right in limited circumstances to require the patent owner tolicense others on reasonable terms as provided for by the terms of GrantNo. DAMD17-96-C-6035 awarded by the Advanced Research Projects Agency(ARPA).

BACKGROUND OF THE INVENTION

[0002] The present invention relates to improved designs for variablecoupler fiberoptic sensors and to sensing apparatus using the improvedsensor designs.

[0003] Variable coupler fiberoptic sensors conventionally employso-called biconical fused tapered couplers. Such couplers aremanufactured by a draw and fuse process in which a plurality of opticalfibers are stretched (drawn) and fused together at high temperature. Theplastic sheathing is first removed from each of the fibers to expose theportions for forming the fusion region. These portions are juxtaposed,usually intertwisted one to several twists, and then stretched whilebeing maintained above their softening temperature in an electricfurnace or the like. As the exposed portions of the fibers arestretched, they fuse together to form a narrowed waist region—the fusionregion—that is capable of coupling light between the fibers. During thestretching process, light is injected into an input end of one of thefibers and monitored at the output ends of each of the fibers todetermine the coupling ratio. The coupling ratio changes with the lengthof the waist region, and the fibers are stretched until the desiredcoupling ratio is achieved—typically by a stretching amount at which therespective fiber light outputs are equal. The coupler is drawn to suchan extent that, in the waist region, the core of each fiber iseffectively lost and the cladding may reach a diameter near that of theformer core. The cladding becomes a new “core,” and the evanescent fieldof the propagating light is forced outside this new core, where itenvelops both fibers simultaneously and produces the energy exchangebetween the fibers. A detailed description and analysis of the biconicalfused tapered coupler has been given by J. Bures et al. in an articleentitled “Analyse d'un coupleur Bidirectional a Fibres OptiquesMonomodes Fusionnes”, Applied Optics (Journal of the Optical Society ofAmerica), Vol. 22, No. 12, Jun. 15, 1983, pp. 1918-1922.

[0004] Biconical fused tapered couplers have the advantageous propertythat the output ratio can be changed by bending the fusion region.Because the output ratio changes in accordance with the amount ofbending, such couplers can be used in virtually any sensing applicationinvolving motion that can be coupled to the fusion region.

[0005] Conventional variable coupler fiberoptic sensors have relied upondesigns in which the fiberoptic coupler is pulled straight, securedunder tension to a plastic support member and, in the resultingpre-tensioned linear (straight) form, encapsulated in an elastomericmaterial such as silicone rubber. The encapsulant forms a sensingmembrane that can be deflected by external forces to cause bending ofthe coupler in the fusion region. The bending of the fusion regionresults in measurable changes in the output ratio of the coupler. Thedisplacement of the membrane can be made sensitive to as little as onemicron of movement with a range of several millimeters.

[0006]FIG. 1 of the accompanying drawings illustrates the basicprinciples of a sensing apparatus including a variable couplerfiberoptic sensor 10 as described above. In the form shown, the sensor10 includes a 2×2 biconical fused tapered coupler 11 produced by drawingand fusing two optical fibers to form the waist or fusion region 13.Portions of the original fibers merging into one end of the fusionregion become input fibers 12 of the sensor, whereas portions of theoriginal fibers emerging from the opposite end of the fusion regionbecome output fibers 14 of the sensor. Reference numbers 18 denote theoptical fiber cores. The fusion region 13 is encapsulated in anelastomeric medium 15, which constitutes the sensing membrane. Thesupport member is not shown in FIG. 1.

[0007] In practice, one of the input fibers 12 is illuminated by asource of optical energy 16, which may be an LED or a semiconductorlaser, for example. The optical energy is divided by the coupler 11 andcoupled to output fibers 14 in a ratio that changes in accordance withthe amount of bending of the fusion region as a result of external forceexerted on the sensing membrane. The changes in the division of opticalenergy between output fibers 14 may be measured by two photodetectors 17which provide electrical inputs to a differential amplifier 19. Thus,the output signal of differential amplifier 19 is representative of theforce exerted upon medium 15. It will be appreciated that if only one ofthe input fibers 12 is used to introduce light into the sensor, theother input fiber may be cut short. Alternatively, it may be retained asa backup in the event of a failure of the primary input fiber. It shouldbe noted that, for simplicity, the coupler 11 is shown without theaforementioned fiber twisting in the fusion region. Such twisting isordinarily preferred, however, to reduce lead sensitivity, which refersto changing of the output light division in response to movement of theinput fiber(s).

[0008] Because variable coupler fiberoptic sensors can be made entirelyfrom dielectric materials and optically coupled to remote electronics,they are particularly advantageous for applications in which thepresence of electrically conductive elements at the sensor locationwould pose the risk of electrical shock, burns, fire, or explosion. Inthe medical field, for example, variable coupler fiberoptic sensors havebeen proposed for monitoring patient heartbeat during MRI examinations.See U.S. Pat. No. 5,074,309 to Gerdt, which discloses the use of suchsensors for monitoring cardiovascular sounds including both audible andsub-audible sounds from the heart, pulse, and circulatory system of apatient. The use of sensing devices having metallic components in an MRIenvironment has been known to cause severe burning of patients due tothe presence of strong radio frequency fields.

[0009] Other applications of variable coupler fiberoptic sensors can befound in U.S. Pat. No. 4,634,858 to Gerdt et al. (disclosing applicationto accelerometers), U.S. Pat. No. 5,671,191 to Gerdt (disclosingapplication to hydrophones), and elsewhere in the art.

[0010] As compared with other types of fiberoptic sensors, the describedvariable coupler sensors offer a uniquely advantageous combination oflow cost, relatively simple construction, high performance (e.g., highsensitivity and wide dynamic range), and versatility of application.Other known fiberoptic sensors have used such principles as microbendingloss, light phase interference, and polarization rotation by means ofbirefringence. Fiberoptic micro-bending sensors are designed to sensepressure by excluding light from the fiber in proportion to the changesin pressure. The output light intensity decreases with increases inmeasured pressure, as pressure is transduced into light loss. Becausethe measurement accuracy is reduced at lower light levels, the dynamicrange of such sensors is severely limited. Interferometric fiberopticsensors measure changes in pressure by applying pressure to an opticalfiber to change its index of refraction. This results in a phase delaythat is measured by utilizing a Mach-Zehnder or Michaelsoninterferometer configuration. These sensors are extremely expensive andrequire sophisticated modulation techniques that render them unsuitablefor many applications. Polarization varying fiberoptic sensors alter thepolarization state of a polarized optical signal in accordance with achange in temperature or pressure. Such polarized light sensors requirespecial optical fiber and expensive polarizing beam splitters.

[0011] Despite their advantages, variable coupler fiberoptic sensorshave been subject to certain limitations inherent in the conventionalpre-tensioned linear (straight) coupler design. The conventional designimposes, among other things, significant geometrical limitations. Inparticular, the size of the sensor must be sufficient to accommodate thefiberoptic leads at both ends of the sensor. The fiberoptic leadarrangement also requires the presence of a clear space around both endsof the sensor in use. Especially in medical applications, such as whenplacing a sensor on a patient's body for continuous monitoring, the sizeand lead positions of the sensor are both important issues. Anotherlimitation results from the fact that any displacement of the fusionregion necessarily places it under increased tension. At some point ofdisplacement, the tension in the fusion region will become excessive,causing the fusion region to crack or break, with resulting failure ofthe coupler.

SUMMARY OF THE INVENTION

[0012] The present invention provides new variable coupler fiberopticsensor designs that have comparatively little or no susceptibility tothe above described over-tensioning of the fusion region. Moreparticularly, in contrast to the conventional pre-tensioned linearcoupler used in prior sensor designs, the present invention utilizes acoupler arrangement that permits deflection of the fusion region withoutaccompanying tension. The coupler may be arranged to have a curved form,for example, which in the most preferred embodiments is substantiallyU-shaped. By arranging the coupler in a substantially U-shaped form, italso becomes possible to locate the fiberoptic leads adjacent to eachother rather than at opposite ends of the sensor, thus avoiding theearlier discussed geometrical limitations inherent in the conventionalpre-tensioned linear coupler design.

[0013] The present invention also retains the basic advantages ofconventional variable coupler fiberoptic sensors relative to other typesof fiberoptic sensors and sensors requiring electrically conductiveelements at the point of use. Indeed, sensors designed in accordancewith the invention may be used to advantage in any application that hasheretofore employed conventional variable fiberoptic coupler sensors.

[0014] Thus, in one of its various aspects, the present inventionprovides a fiberoptic sensing apparatus comprising a fiberoptic couplerin which a plurality of optical fibers are joined through a fusedcoupling region. The optical fibers include at least one input opticalfiber and a plurality of output optical fibers, the fiberoptic couplerdistributing light incident to the input optical fiber among theplurality of output optical fibers. The apparatus further comprises asupport member, and the coupler is mounted to the support member andconfigured such that at least a portion of the fused coupling region canbe deflected without putting the fused coupling region under tension.

[0015] In another of its aspects the invention provides a fiberopticsensing apparatus comprising a fiberoptic coupler in which a pluralityof optical fibers are joined through a fused coupling region. Theoptical fibers include least one input optical fiber and a plurality ofoutput optical fibers, the fiberoptic coupler distributing lightincident to the input optical fiber among the plurality of outputoptical fibers. The apparatus further comprises a support member, andthe coupler is mounted to the support member and configured such thatthe fused coupling region has substantially a U-shape.

[0016] Other aspects of the invention will become apparent from areading of the following description of the preferred embodiments withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 illustrates the basic construction of a conventionalvariable coupler fiberoptic sensor.

[0018]FIG. 2 is a top view of a variable coupler fiberoptic sensoraccording to the invention, being designed for monitoring cardiovascularsounds such as from the wrist or the chest.

[0019]FIG. 3 is a sectional side view of the sensor of FIG. 2.

[0020]FIG. 4 shows explanatory views (Views 4 a-4 d) of normal anddeflected states of the fusion region of a conventional pre-tensionedlinear coupler.

[0021]FIG. 5 shows corresponding explanatory views (Views 5 a-5 d) for aU-shaped fusion region in accordance with the principles of theinvention.

[0022]FIG. 6 shows a fiberoptic wrist sensing apparatus according to theinvention.

[0023]FIG. 7 is a graph depicting the response of the wrist sensor inthe apparatus of FIG. 6 to pulsations of the wrist.

[0024]FIG. 8 is another graph of the sensor response at the wrist.

[0025]FIG. 9 is an exploded view of another wrist sensor according tothe invention.

[0026]FIG. 10 is an end view of the FIG. 9 sensor in assembled form.

[0027]FIG. 11 illustrates another wrist sensor according to theinvention, shown in section as worn on the wrist.

[0028]FIG. 12 is a perspective view of a carotid artery sensor accordingto the invention.

[0029]FIG. 13 is a fragmentary side elevation of the FIG. 12 sensor.

[0030]FIG. 14 is a perspective view showing the FIG. 12 sensor and itsfiberoptic leads with installed connectors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031]FIGS. 2 and 3 of the accompanying drawings show an example of avariable coupler fiberoptic sensor 20 in accordance with the invention.The sensor is constructed for placement against a medical patient'sbody, such as on the chest or wrist, for sensing cardiovascular soundsincluding both audible and sub-audible sounds from the heart, pulse, andcirculatory system.

[0032] The sensor 20 comprises a support member 22 having a generallycircular head portion 24, which is provided with a central well orthrough hole 26, and a handle-like extension 28. A biconical fusedtapered coupler 30 is mounted to the support member with at least aportion (here, the entirety) of its fused coupling region 32 disposed inthe space 26 and arranged in a U-shape. Input fiber leads 34 and outputfiber leads 36 of the coupler are disposed beside one another in achannel 29 formed in the extension 28. The leads are manipulated so asto bend the coupling region 32 through 180° into the desired shape andthen secured within the channel by a suitable adhesive, such as anepoxy-based glue. The coupling region, which is not under tension, maybe potted by filling the space 26 with elastomer to form a sensingmembrane 38 (not shown in FIG. 2) in the known manner—for example, byfilling with a silicone rubber such as GE RTV 12. Alternatively, as willbe seen hereinafter, the coupling region may be coated with a layer ofcoating material such as GE SS 4004 (polydimethylsiloxane with methylsilsesquioxanes) to eliminate the need for potting. This material isnormally used as a primer for bonding room temperature vulcanizing (RTV)materials to surfaces that would otherwise form weak bonds. Theadvantage of eliminating the potting is that the sensitivity isincreased, because the potting tends to reduce sensitivity no matter howthinly it is applied. Support member 22 is suitably formed of a moldableplastic, such as Plexiglass®, polyvinyl chloride (PVC), or othersuitable materials known in the art.

[0033] As shown in FIG. 3, the upper portion of the membrane 38 has aconvex surface 39 that protrudes from the plane of the support structurefor contacting the patient's body. The convex configuration of thecontact surface makes the sensor more of a point probe to betterlocalize the cardiovascular sounds being monitored. In a practicalembodiment of the sensor, the maximum diameter of the membrane may beabout the same as that of a nickel with the contact surface protrudingby about half that amount, but the membrane may be smaller or larger asdesired to suit a particular application. The support plate dimensionsmay be any convenient size, so long as the coupler fusion region and thefiber portions near the fusion region are securely supported. Thesensitivity of the device is dependent upon the stiffness of themembrane, as in prior devices. Using softer or stiffer membranematerials will provide different sensitivities to the readings receivedfrom the body coupling.

[0034] When the contact surface 39 is placed upon a patient's body, asat the chest or wrist, the membrane 38 couples skin displacementsassociated with cardiovascular sounds to the coupling region 32 of thefiberoptic coupler 30. The coupling region is thereby deflected,changing the light output ratio of the output fibers 36 in accordancewith the sounds being monitored.

[0035]FIGS. 4 and 5 provide a pictorial comparison between thedeflection of a conventional pre-tensioned linear fiberoptic coupler andthe deflection of the U-shaped coupler in the sensor of FIGS. 2 and 3.Views 4 a and 4 c are top and side views, respectively, showing thefusion region of the conventional coupler in its normal state. Views 4 band 4 d are corresponding views of the fusion region being deflected bya downward force F. Views 5 a-5 d in FIG. 5 are corresponding views toFIG. 4, but show the U-shaped coupler employed in the present invention.

[0036] As will be appreciated from View 4 d, the deflection of thefusion region in the conventional coupler causes a bowing that tends tostretch and thereby increase the tension on the fusion region. Bycontrast, the deflection of the U-shaped fusion region in View 5 d,which is seen to occur along a direction perpendicular to the plane ofthe U-shape, merely causes a flexing of the U along its height(horizontal dimension in View 5 d), with the fusion region being undersubstantially no tension. Thus, even large displacements of the fusionregion will not cause cracking or breaking.

[0037] In the practice of the present invention, the fusion region ofthe fiberoptic coupler need not be U-shaped or even substantially so.Any configuration of the fusion region that allows for deflectionwithout tensioning of the fusion region may be used. For example,arcuate shapes, parabolic and hyperbolic shapes with widely divergentsides, and other curved shapes may be used. A substantially U-shapedform may be preferred, however, because such a form allows positioningof the coupler input and output leads on the same side of the sensor,resulting in a compact design that does not require clearances toaccommodate leads at opposite ends of the structure. A substantiallyU-shaped form also produces significant increases in sensitivity,linearity, and dynamic range as compared with the conventional linearcouplers. Other shapes, such as those mentioned above, may also producethese advantages. The length of the fusion region may be establishedduring production as previously explained, and in actual sensorsconstructed in accordance with the invention has typically been about1.5 cm.

[0038]FIG. 6 shows another variable coupler fiberoptic sensor 20′ inaccordance with the invention. The sensor has the same basic structureas that of the previous embodiment, except that the support member 22′is formed as a substantially rectangular plate angled at about 30° toconform to the human wrist anatomy and facilitate wearing of the sensorby the patient, as by strapping the sensor to the wrist. As also shownin FIG. 6, one of the input leads 34 is optically coupled to a lightsource 40 (e.g., and LED or semiconductor laser) and the output fibers36 are optically coupled to a photodetection/differential amplifiercircuit 42, as previously described in connection with FIG. 1. Thedifferential amplifier circuit may be coupled to an oscilloscope 44 orsome other form of display device, such as a personal computer, todisplay the output of the differential amplifier circuit. If appropriateto a particular application, the support member may house the lightsource 40, the differential amplifier circuit 42, and a radiotransmitting device (not shown) coupled to the differential amplifiercircuit to provide for remote monitoring. Indeed, such provision can bemade in any of the sensor structures of the invention described herein.

[0039]FIG. 7 shows the wrist heartbeat/breathing signal obtained from ahuman subject with the sensor 20′ of FIG. 6. The data stream in FIG. 7was obtained at a sampling rate of 128 samples per second. It will beappreciated that the pulse waveform, as read by the sensor, is a morecomplex phenomenon than standard pulse readings. The pulse waveformexhibits the amplitude structure of the pulse as a function of time. Theamplitude structure of the pulse is not what is “felt” as an impulsefunction by a finger at a pulse point, although that function ispresent. Within the amplitude structure, there are all of the heartsounds as well as information on breathing and other indicators ofphysical condition. The sensitivity achieved with sensors according tothe present invention makes them very good at sensing the complex pulsewaveform.

[0040]FIG. 8 shows another wrist heartbeat/breathing signal obtainedfrom a human subject with the sensor 20′. Here, the data stream wasdigitized using a 12-bit A/D converter at a sampling rate of 64 samplesper second. The heartbeat signal is very well resolved, as the insetgraph demonstrates. In addition, the modulation introduced by thebreathing cycle is clearly visible over the course of the 84 second run.

[0041]FIGS. 9 and 10 show another embodiment of the invention, appliedto a wrist sensor 50. In this embodiment, the fusion region 62 of thefiberoptic coupler is not potted, but coated as previously discussed inconnection with FIG. 1. The fusion region 62 is coupled to pulsations ofthe wrist (denoted by arrow P) by a fluid- or gel-filled elastic pillow68. The fiberoptic coupler is mounted to a support plate 52 similar tothat of FIG. 6, except that the support plate 52 is planar, not angled(the channel for the input and output leads 64, 66 having been omittedfrom illustration for simplicity). The support plate is secured to thetop side of pillow 68 and a cover 69 is attached to the top side of thesupport plate to protect the fusion region 62 of the coupler 60 at thehole 56. The hole 56 allows the hydraulic pressure of the pulse activityto push on and deflect the fusion region by virtue of the contactbetween the fusion region and the upper surface of the pillow 68 which,due to its flexibility, protrudes into the hole 56 to contact thecoupler fusion region. A wrist strap 57 attached to the support plate52, as by glue, allows the sensor to be secured to the wrist. Referencenumbers 64 and 66 denote the input fibers and output fibers,respectively.

[0042] The unpotted sensor design of FIGS. 9 and 10 is advantageous overthe potted designs previously described, because the absence of thesensing membrane results in greater sensitivity. Also, unlike the bentdesign in FIG. 6, the planar configuration of the support plate does notrequire out-of-plane bending of the coupler leads, which causes areduction of light intensity. Instead, the coupler is maintained in aplanar configuration, which optimizes the light intensity in the system.

[0043]FIG. 11 shows still another embodiment of the invention, appliedto a wrist sensor 70 shown in cross-section as worn on the wrist. Thesensor includes a frame member 72 having an inner configuration whichconforms generally to the wrist, as shown. The frame member may beconstructed from any suitable material, preferably a plastic such asDelrin®, PVC, acrylic, Lucite®, Plexiglass®, styrene, or other polymers.

[0044] An upper portion of the frame provides a chamber 77 for housingthe fiberoptic coupler 80 and its support plate 81. Since the coupler ishoused by the frame member, the support plate, which is channeled toreceive the input and output leads, need not include an opening (e.g., awell or through hole) to house the fusion region 82 of the coupler as inprevious embodiments. The fusion region is coated, rather than potted,as previously described. The support plate 81, which may be of the samematerial as the frame 72, and the coupler are assembled as a module andglued in place in the chamber 77. The chamber is closed by a protectivecover plate (not shown).

[0045] To couple the fusion region to the pulsations of the radialartery, a fluid column 74 is provided. The column has a pair ofresilient membranes 73 and 75 provided at its inner and outer ends,respectively, and extends through the thickness of the frame 72 betweenthe chamber 73 and the frame inner surface. The coupler module isinstalled with the coupler fusion region 82 in contact with the outermembrane 75 of the fluid column. The outer membrane is attached to anannular boss 76 to raise the height of the fluid column for contact withthe coupler fusion region.

[0046] The contact with the outer membrane may subject the fusion regionto a slight pre-load. The coupler may be manufactured such that thepre-loading of the fusion region will produce a substantially equaldivision of light between the output fibers, thus providing a morelinear dynamic range. The inner portion (lower portion in FIG. 11) ofthe fluid column is stepped as shown, so as to increase the diameter ofthe coupling area at the wrist.

[0047] The membranes constitute an important part of the fluid column.Since the arterial pulsations are weak, the membranes should be light,thin, and of low durometer and high extensibility for optimumperformance. At the same time, at least the inner membrane should berugged enough to endure continuous contact with the skin. A materialfound to have excellent characteristics for the membrane is FlexChem, anFDA-approved, highly durable, vinyl based material available in pelletform from Colorite. FlexChem is also thermo-moldable, which permits theinner sensing membrane 73 to be molded to provide maximum coupling areawith the radial artery and to protrude from the inner surface of theframe member 72 for better coupling with the wrist. A compatible fluidfor use with FlexChem membranes is medical grade MDM silicone fluidavailable from Applied Silicone Corp. Water, incidentally, is notpreferred for use with FlexChem membranes since the membranes arepermeable to water vapor.

[0048] Several inner membrane sizes were tested to determine the effecton sensor response. In particular, membrane diameters of 4 mm, 7 mm, and10 mm were tested for response to driven-oscillator stimuli calibratedusing a commercial accelerometer. The response was examined over afrequency range of 0 to about 11 Hz (cardiovascular and breathingsignals are typically in the range from 0.1 to 4 Hz). Each of themembranes provided acceptable response, with the 10 mm membraneproviding the best response.

[0049] Returning to FIG. 11, the present embodiment also demonstrateshow ancillary components, such as the light source and output circuitry(e.g., photodetectors and differential amplifier circuitry) may beincorporated into the sensor unit. More particularly, such componentsmay be housed in one (as shown) or more internal chambers 79 of theframe 72.

[0050] FIGS. 12-14 illustrate another embodiment of the invention,applied to a sensor 80 for the carotid artery. This sensor uses aplanar, channeled support plate 82 and coupler arrangement similar tothat of FIG. 9, except that the fusion region is potted to provide asensor membrane. The membrane area may be made sufficiently large (e.g.,about the size of a quarter dollar) to allow for the addition of aspherical cap 99′ over the convexly protruding surface of the sensingmembrane 98. The addition of the spherical cap renders the sensor lesssensitive to any rocking motion caused by the hand when the sensor ismanually pressed against the neck. The coupler is protected at the backside (bottom in FIGS. 12 and 13) of the sensor by a plastic cover plate97. The sensor may be secured to the neck by any suitable means, such asadhesive tape.

[0051] The input and output fibers are encased as pairs in respectiveprotective sheaths 102 and 104, which in turn are encased in an outerprotective sheath 106. Fiberoptic connectors 108 are provided at theends of the leads to interface the sensor with external components.

[0052] It should be noted that the optical fiber used in the sensors ofthe present invention is most preferably of very high quality, such asCorning SMF28 which exhibits an optical loss of about 0.18 dB per Km.The photodetectors may be gallium-aluminum-arsenide or germaniumdetectors for light wavelengths above 900 nm and silicon detectors forshorter wavelengths.

[0053] The photodetectors may be connected in either a photovoltaic modeor a photoconductive mode. In the photovoltaic mode, transimpedanceamplifiers (which convert current to voltage) may be used to couple thedetectors to the differential amplifier inputs. The transimpedanceamplifier outputs may also be filtered to eliminate broadband noise. Asanother example, the transimpedance amplifier outputs could be feddirectly to a digital signal processor (DSP), where subsequent filteringand processing could be accomplished digitally through programmedalgorithms. A DSP having transimpedance inputs would, of course,eliminate the need for discrete transimpedance amplifiers in thepreceding example. In the photoconductive mode, the detector outputs canbe connected to a conventional voltage amplifier. This approach resultsin more noise, but may be used in applications where cost is a majorconcern and a lower noise level is not.

[0054] It is to be understood, of course, that the foregoing embodimentsof the invention are merely illustrative and that numerous variations ofthe invention are possible in keeping with the invention as more broadlydescribed herein.

What is claimed is:
 1. Fiberoptic sensing apparatus, comprising: afiberoptic coupler in which a plurality of optical fibers are joinedthrough a fused coupling region, said optical fibers including at leastone input optical fiber and a plurality of output optical fibers, saidfiberoptic coupler distributing light incident to said input opticalfiber among said plurality of output optical fibers; said coupler beingsupported in a configuration such that at least a portion of saidcoupling region is curved and can be deflected to change the lightdistribution among said output fibers with said coupling region beingunder substantially no tension.
 2. Fiberoptic sensing apparatusaccording to claim 1, further comprising a device optically coupled tosaid plurality of output fibers to detect the change of lightdistribution.
 3. Fiberoptic sensing apparatus according to claim 2,further comprising a display connected to an output of said device. 4.Fiberoptic sensing apparatus according to claim 1, wherein said inputand output optical fibers are single mode optical fibers.
 5. Fiberopticsensing apparatus, comprising: a fiberoptic coupler in which a pluralityof optical fibers are joined through a fused coupling region, saidoptical fibers including at least one input optical fiber and aplurality of output optical fibers, said fiberoptic coupler distributinglight incident to said input optical fiber among said plurality ofoutput optical fibers; said coupler being supported in a configurationsuch that said fused coupling region has substantially a U-shape ofwhich at least a portion can be deflected to change the lightdistribution among said output optical fibers.
 6. Fiberoptic sensingapparatus according to claim 5, further comprising a device opticallycoupled to said plurality of output fibers to detect the change of lightdistribution.
 7. Fiberoptic sensing apparatus according to claim 6,further comprising a display connected to an output of said device. 8.Fiberoptic sensing apparatus according to claim 5, wherein said inputand output optical fibers are single mode optical fibers.